my kids got a a couple of nexus 7s for their xmas, and i preloaded it with some free apps. i tied them to the same google account, and would now like to purchase some apps for them, but i dont want them to be able to randomly head to the app store and purchase apps from my google wallet which i believe is automatically saved following my first purchase.

Go into Play Store, select the menu button (top right). Goto settings and set a pin.
You will then need to enter pin to purchase any app (free or paid). You can also set content filtering so your kids cannot see apps in play store.

One other option is to use the Safe Browser app from Mobicip. It has a built-in Restrictions feature that allows you to password-lock other apps including the Google Play Store, the default browser, other browsers and apps as you see fit. The safe browser creates a safe and secure internet environment for young users. You may find this tutorial to be helpful to set this up: How To Setup Parental Controls On Android

my kids got a a couple of nexus 7s for their xmas, and i preloaded it with some free apps. i tied them to the same google account, and would now like to purchase some apps for them, but i dont want them to be able to randomly head to the app store and purchase apps from my google wallet which i believe is automatically saved following my first purchase.